Suspension of micro- and nanoparticles are ubiquitous in our environment both naturally formed, such as microorganisms, including viruses, bacteria, animal cells, proteins, etc., or synthetically manufactured, such as colloidal latexes, paints, pigments and a variety of metallic or semiconductor nanoparticles for industrial applications. The ability to control the strength of particle-to-particle interactions and the stability of the particles in suspension are critical for a variety of industries that process nanoparticles, whether as raw materials, manufacturing intermediates, or final products. One of the most important parameters during the processing of these particulate suspensions is measuring and controlling of colloidal stability against fluctuation or aggregation.
Control of suspension stability is a complex task involving parameters such as salt concentration, pH, solvent conditions, surface charge of the particles, and the type and quantity of surfactants in the solution. Most of these adjustable parameters are configured to fine tune particle-to-particle interactions. The stronger the repulsive interaction between the particles, the more stabile the suspension is. In industrial practice, in-line monitoring of colloidal stability is difficult to implement because traditional monitoring instruments are quite complex. More often, the stability of colloidal suspensions against mechanical agitation or chemical variations (e.g., pH and salt) are tested in batches where samples are removed from a reactor. In such cases, the transition from a desired stable phase to the flocculated phase can be monitored by any of visual inspection (non-homogeneous texture), light transmission, or scattering techniques (degree of opacity). However, these monitoring techniques provide limited results and represent the stability of sample that may have altered after removal from the reactor. Accordingly, a continuing and unmet need exists for new and improved methods that can be use to study nanoparticles in solution without mechanical or chemical interventions, especially nano-scale methods. Using light to create mechanical force to manipulate the physical behaviors of particles in the medium surrounding them is a desirable means for studying particle-to-particle interactions in the medium. Such parameters may be measured in situ without needing to interrupt a process or remove the solution from the batch process.